1. Field of the Invention
The present invention relates to mobile communication systems and code division multiple access (CDMA) technology, and more particularly, to an overhead message and a channel hashing method using the same, in which new fields are assigned for channel hashing.
2. Discussion of the Related Art
The base stations of a mobile communication system typically support a plurality of frequency assignments. To inform a mobile station (e.g., cellular telephone) of the supportable frequency assignments, a base station generates and sends to the mobile station a CDMA channel list message (CCLM) or an extended CDMA channel list message (ECCLM), both of which are types of overhead messages sent out over a forward paging channel or a forward broadcast control channel. An overhead message is a message sent by a base station to all mobile stations, to establish system operating parameters by communicating information that may be specific to the base station or may be common to the entire system.
Generally speaking, only one or the other of the CCLM and ECCLM is received by a mobile station at any given time, since IS-95A and IS-95B systems send out a CCLM while IS-2000 systems send out an ECCLM. Thus, the CCLM normally applies where the ECCLM is unavailable.
The CCLM or ECCLM contains a frequency assignment list (or CDMA channel list) of all supportable frequency assignments for the receiving mobile station. Based on the received information on the frequency assignments, as well as data extracted from its own telephone number, the mobile station selects one service frequency assignment among the list. The process of determining one frequency assignment for selection and service is called “channel hashing.”
A channel hashing method according to a related art will be explained with reference to FIG. 1, wherein a mobile station receives an overhead message via a forward paging channel (F-PCH).
In a step 101, when a mobile station is powered up (turned on) or its system status is changed, the mobile station first selects an internal system for operation, namely, a CDMA or analog system. In doing so, the mobile station performs a call processing preparation procedure using a primary channel of the selected internal system, and if the primary channel fails, a secondary channel is used. Subsequently, the mobile station sequentially performs operations for acquiring first a pilot channel and then a synchronization channel.
The synchronization channel carries system information and timing information, for creating a system parameter message (SPM), which is an overhead message sent out by a base station and includes a CDMA_FREQ field. In specially supported systems (described later), the SPM may include an EXT_CDMA_FREQ field in addition to a CDMA_FREQ field. Thus, when transferring an overhead message, the system information indicates the availability of a primary paging channel in a prescribed frequency assignment.
If the mobile station fails to acquire either a pilot channel or a synchronization channel, the processes of the step S101 are performed anew, starting from system selection. The next step is performed only upon acquisition of the pilot and synchronization channels, and in particular, the system and timing information of the synchronization channel, which are necessary to determine a frequency assignment for a primary paging channel.
In a step S102, the mobile station receives the SPM, which includes the frequency assignment for the primary paging channel. That is, the mobile station first receives, via the synchronization channel, frequency assignment (FA) information from the base station. The FA information thus informs the mobile station of the primary paging channel, so that the mobile station may tune to the assigned frequency of the primary paging channel and thereby receive the SPM. The received SPM contains information on the transmission of an overhead message (e.g., the CCLM or ECCLM), indicating transmission through a corresponding paging channel or broadcast control channel.
A step 103 determines whether the SPM indicates the transmission of an ECCLM or a CCLM. The mobile station will use the FA information (CDMA channel list) included in the ECCLM or CCLM to select a service frequency assignment and thus perform channel hashing. Accordingly, the mobile station will receive either the CCLM in a step S104 or the ECCLM in a step S105.
Referring to a step S106, where the FA information of the ECCLM has been received and is to be used for channel hashing, the mobile station determines whether the system, i.e., the base station and the mobile station, has special system support which includes support for a quick paging channel (QPCH) or capability of a radio configuration beyond RC2 (>RC2). If the above special system support is provided, the mobile station selects in a step S107 a CDMA channel (service frequency assignment) from a subset of channels among the frequency assignment list included in the ECCLM. Here, the subset of channels includes only those channels that provide support for quick paging or a radio configuration beyond RC2. On the other hand, if the mobile station uses the FA information of a CCLM for the channel hashing, or if despite receiving an ECCLM, there is no special system support as above, the mobile station selects in a step S108 the service frequency assignment from the entire frequency assignment list included in the CCLM or ECCLM.
Accordingly, in channel hashing as above, the mobile station uses the information of the CCLM or ECCLM to select a service frequency assignment from the entire CDMA channel list such that, in the case of an ECCLM, one frequency assignment is selected from a predetermined subset. Such channel hashing in a 1×EV-DV (1×evolution—data and voice) system, however, is based on a random selection from the entire frequency assignment list, even when an ECCLM is sent, so that the selected frequency assignment may provide no 1×EV-DV support. Furthermore, a forward packet data channel (F-PDCH) cannot be assigned to a mobile station of the 1×EV-DV system. Therefore, the channel hashing method according to the related art is a hindrance to the development and proliferation of packet data supportable systems.